Ultraviolet light emitting devices having compressively strained light emitting layer for enhanced light extraction

ABSTRACT

Light emitting devices having an enhanced degree of polarization, P D , and methods for fabricating such devices are described. A light emitting device may include a light emitting region that is configured to emit light having a central wavelength, λ, and a degree of polarization, P D , where P D &gt;0.006λ−b for 200 nm≤λ≤400 nm, wherein b≤1.5.

RELATED PATENT DOCUMENTS

This application is a continuation of U.S. patent application Ser. No.13/328,783, filed Dec. 16, 2011, now U.S. Pat. No. 9,252,329, which is acontinuation-in-part of U.S. patent application Ser. No. 13/252,691,filed Oct. 24, 2011, which are incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT

This invention was made with Government support under U.S. ArmyCooperative Agreement No. W911NF-10-02-0102 awarded by the DefenseAdvanced Research Projects Agency (DARPA). The Government has certainrights in this invention.

SUMMARY

Light emitting devices having an enhanced degree of polarization, P_(D),and methods for fabricating such devices are described. In someembodiments, a nitride-semiconductor light emitting device comprising alight emitting region is configured to emit light having a centralwavelength, λ, and a degree of polarization, P_(D), where P_(D)>0.006λ−bfor 200 nm≤λ≤400 nm, wherein b≤1.5. In some cases, b is equal toabout 1. For example the light emitting region may comprise at least oneof GaN, InAlN, AlGaN, InGaN and InAlGaN.

Some embodiments involve a light emitting device having a light emittingregion grown above a bulk crystalline AlN substrate, wherein variationin reciprocal lattice values of the AlN substrate and the light emittingregion is less than about 1.5%.

Some embodiments involve a light emitting device comprising a lightemitting region configured to emit light having a central wavelength, λ,and a degree of polarization, P_(D), where P_(D)>0 for 200 nm≤λ<300 nm.

Some embodiments involve methods for making light emitting devices. Forexample, a method may include growing a first heterostructure and asecond heterostructure. A III-nitride light emitting region is grown onthe first heterostructure so that the light emitting region is disposedbetween the first and second heterostructures. The light emitting regioncomprises at least one compressively stained layer, wherein acompressive strain, ε_(α), in the compressively strained layer satisfiesthe inequality ε_(α)<−0.00615+0.00023*(λ(nm)−230 nm).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram that illustrates TE and TM polarized light;

FIGS. 2 and 3 show the relative energies of valence bands inAl_(x)Ga_(1-x)N calculated as a function of quantum well aluminum molarfraction;

FIG. 4 shows the degree of polarization of various devices having lightemitting regions comprising III-nitride alloys, including a device thathas a compressively strained light emitting region according toembodiments discussed herein;

FIG. 5 shows the photoluminescence plot for a device having acompressively strained light emitting region according to embodimentsdiscussed herein;

FIG. 6 is a diagram illustrating a light emitting layer disposed betweentwo barrier layers according to embodiments discussed herein;

FIG. 7 illustrates the compressive strain that may be used to achievedominant TE polarization at the specified wavelength for five barrierlayer Al compositions according to embodiments discussed herein;

FIG. 8 illustrates the difference between the lattice constant of thequantum wells and the lattice constant of the substrate,a_(strained)-a_(bulk-AlN), that achieves an enhanced degree ofpolarization for two barrier layer Al compositions according toembodiments discussed herein;

FIG. 9 illustrates a light emitting device having a light emittingregion comprising one or more compressively strained Al_(x)Ga_(1-x)Nlayers disposed between Al_(y)Ga_(1-y)N barrier layers andheterostructures disposed on either side of the light emitting region toenhance the crystal quality of the epitaxially grown layers of the lightemitting region according to embodiments herein;

FIG. 10 illustrates a device that includes a first heterostructurehaving a transition layer according to embodiments discussed herein;

FIG. 11 shows the various layers and layer compositions of an example ofa light emitting device in accordance with embodiments described herein;

FIGS. 12 and 13 show surface atomic force microscopy (AFM) scans of theheterostructure of FIG. 11 grown up to the n-contact layer, without(FIG. 12) and with (FIG. 13) transition layers;

FIG. 14 illustrates a Al_(0.7)Ga_(0.3)N layer grown over a bulkcrystalline AlN substrate with two 38 nm transition layers comprisingshort period superlattices (SPSLs) disposed between the AlN substrateand the Al_(0.7)Ga_(0.3)N layer in accordance with some embodiments;

FIG. 15 shows an X-ray reciprocal space map of the structure shown inFIG. 14;

FIG. 16 shows the polarization-resolved spectrum of a λ=279 nm laserexhibiting an enhanced degree of polarization in accordance withembodiments described herein;

FIG. 17 shows the polarization-resolved spectrum of a λ=290 nm laserexhibiting an enhanced degree of polarization in accordance withembodiments described herein;

FIG. 18 shows the layers of a light emitting diode emitting light atλ=291 nm with enhanced P_(D) light emission according to embodimentsdiscussed herein; and

FIG. 19 is a graph that shows the relationship between Al content andwavelength for Al_(x)Ga_(1-x)N.

DESCRIPTION OF VARIOUS EMBODIMENTS

Light generated by a semiconductor light emitting region may havepolarization characteristics that affect the direction of propagationfrom the light emitting device. As illustrated in FIG. 1, light thatpropagates along an axis perpendicular to the emitting surface 105 ofthe light emitting device 101, or at an angle with respect to the axisperpendicular to the emitting surface 105 that is within an escape cone102 of emitted light, emerges from the emitting surface 105 of thedevice 101. Light that propagates parallel to the emitting surface 105of the device 101, or at an angle outside of the escape cone 102 may besubstantially lost to absorption within the device 101 withoutcontributing to the total light output of the device 101.

As further illustrated in FIG. 1, a light emitting device 101 cangenerate TM polarized light (having an electric field generallyperpendicular to wafer plane) and TE polarized light (having an electricfield generally parallel to the wafer plane of the device). TM polarizedlight propagates in a direction generally parallel to the wafer plane,i.e. generally, parallel to the emitting surface 105 of the lightemitting device 101. TE polarized light propagates in a directiongenerally perpendicular to the wafer plane, i.e., generallyperpendicular to the emitting surface 105 of the light emitting device101. Thus, in the example of FIG. 1, the TE polarized light can emergeeasily from the device, whereas the TM polarized light is predominantlytrapped and absorbed within the device layers.

Ultraviolet light emitting devices, such as ultraviolet light emittingdiodes (UV LEDs) have been fabricated using III-nitride materialsystems, such as AlGaN, InAlN and/or InAlGaN. For devices that useIII-nitride materials, the wavelength of the emitted light decreases andthe amount of TM polarized light generated increases with increasing Alcontent (Al molar fraction). Thus, for UV LEDs, as the Al content in thelight emitting region increases, the fraction of TM polarized lightemitted by the light emitting region also increases, which can come atthe expense of emitted TE polarized light. Consequently, the lightoutput efficiency of UV LEDs may be limited by light extractionefficiency as the wavelength of the devices decreases.

For some device configurations, TE polarized light is propagatedgenerally along the axis perpendicular to the output surface of thedevice and TM polarized light is propagated generally along an axisparallel to the output surface of the device. Thus, the relative amountof light generated by a light emitting region of a device having thepreferred (TE) polarization, P_(D), referred to herein as the degree ofpolarization, can be characterized by the equation:

$\begin{matrix}{{P_{D} = \frac{I_{TE} - I_{TM}}{I_{TE} + I_{TM}}},} & \lbrack 1\rbrack\end{matrix}$

where I_(TE) and I_(TM) are the integrated in-plane-emitted TE and TMpolarized electroluminescence light intensities, respectively. Asdiscussed above, it is desirable to enhance the amount of TE lightgenerated by the device to increase the overall light extractionefficiency of the device.

III-nitride materials are anisotropic along and perpendicular to thedirection of the crystallographic axis. This anisotropy results in thesplitting of the valence bands. For example, the splitting of thevalance bands in III-nitride alloys produces a degree of polarization ofthe emitted light that changes with the Al composition of the alloy. Forrelatively high Al concentrations used in short wavelength (<about 300nm) devices, the TM-polarized light can dominate. However, the valenceband energies are also affected by the lattice strain, which influencesthe degree of polarization. Compressive strain is associated withincreased generation of TE polarized light, whereas tensile strain isassociated with increased generation of TM polarized light. Thus,growing devices having III-nitride light emitting regions that arecompressively strained increases the relative amount of TE-polarizedlight.

Various examples provided herein are based on epitaxially grown layerswith (0001) c-plane crystallographic orientation, and involve enhancingthe degree of polarization, P_(D) to achieve an increase in lightpropagated along the direction perpendicular to the device outputsurface. However, the general concepts involving enhancing the emittedTE polarized light as discussed herein can be applied to devices havinglayers grown having other crystallographic orientations.

Light emitting device structures that produce an enhanced degree ofpolarization, P_(D), are discussed along with methods to produce thesedevice structures. In some embodiments, the light emitting region of thedevice is grown under compressive strain. The compressive strain altersthe semiconductor band structure of the light emitting layers, resultingin devices that exhibit an enhanced degree of polarization.Alternatively or additionally, the Al content of barrier layers of inlight emitting region may be selected to increase the generation of TEpolarized light.

In addition to the configuration of the light emitting region discussedherein, some light emitting devices include heterostructures disposed onone or both sides of a light emitting region. These heterostructurescontribute to device performance by facilitating the growth of highquality crystalline layers, including the barrier layers and QWs, byreducing defects, cracking, and surface roughness, for example. One ormore of the heterostructures may include transition layers that serve togradually transition the device layers from the lattice structure of thesubstrate to that of the light emitting region. These transition layersinfluence the crystal quality and overall reliability of the lightemitting device. In various embodiments, the light emitting regionsdiscussed herein can be used alone or in combination withheterostructures and/or transition layers as discussed in more detailbelow. In various embodiments, the heterostructures and light emittingregion can be grown on substrates comprising sapphire, GaN, AlN, AlGaN,InAlGaN, Si, SiC and/or other substrate materials.

FIGS. 2 and 3 show the relative energies of valence bands inAl_(x)Ga_(1-x)N calculated as a function of quantum well aluminum molarfraction. The valence band energies depend on the strain in the emittingAl_(x)Ga_(1-x)N material. The strain can be controlled by pseudomorphicgrowth of the light emitting region on various substrates. FIG. 2 showsthe valence bands in Al_(x)Ga_(1-x)N material grown on a bulkcrystalline AlN substrate and strained to the lattice constant of theAlN. FIG. 2 shows the bands in Al_(x)Ga_(1-x)N material grown onAl_(0.7)Ga_(0.3)N and strained to the lattice constant of theAl_(0.7)Ga_(0.3)N. Theoretically, TM-polarization occurs when thehighest valence band has “p_(z)” character, and TE polarization occurswhen the highest valence band has “p_(x),p_(y)” character. In FIG. 2,this particular calculation indicates a crossover composition at about60% Al. The TM polarized emission becomes dominant for Al compositionsgreater than about 60%. In FIG. 3, the calculation indicates across-over composition at about 43% Al. The TM polarized emissionbecomes dominant for Al compositions greater than about 43%. The generaltrend is that the crossover composition shifts to higher Al compositionsas the compressive strain in the emitting material is increased. Thecalculations shown in FIGS. 2 and 3 do not take into account the factthat the holes are confined within quantum wells of finite width andthat there are electric fields in these quantum wells which arise fromspontaneous and piezoelectric polarization. Thus these figures areintended to describe a general trend but not necessarily to provide theactual cross-over wavelength, which, for various device structures, maydiffer from those indicated in the figures. Regardless of the specificcrossover point, FIGS. 2 and 3 illustrate that pseudomorphic growth ofAlGaN on AlN produces more compressive strain in the light emittinglattice which allows more TE polarization to be achieved at relativelyshorter wavelengths than can be achieved in growth on Al_(0.7)Ga_(0.3)N,which produces a light emitting region that is relatively lesscompressively strained.

To determine the actual cross-over wavelength, we have performedcalculations of the energies of the hole states in Al_(x)Ga_(1-x)Nquantum wells surrounded by barrier regions of Al_(y)Ga_(1-y)N. Thesecalculations take into account the well width and the electric fields inthe wells. Both the quantum well and the barrier region have the samein-plane lattice constant, a_(strained), which may be equal to or largerthan the in-plane lattice constant of bulk AlN, a_(bulk-AlN). The Alcomposition y of the barrier material is greater than x so that theholes are confined within the quantum wells. The calculations that wehave performed show that increasing y promotes TE polarization. Therelative energies of the hole states depend on y and x. As the barriercomposition y increases the value of x for which the polarizationcrosses over from TE to TM is increased. Thus, in addition to thecompressive strain in the light emitting region, the Al composition ofthe barrier is a factor affecting the polarization of the light.

FIG. 4 shows the measured the degree of polarization, P_(D), as setforth in Equation 1 above, obtained from light emitting diodes (LEDs)having central wavelengths ranging from λ=250 nm to λ=390 nm. Thecentral wavelength, λ, is the wavelength emitted from the device thathas the greatest intensity. Sometimes the central wavelength of thedevice is more simply referred to herein as the emission wavelength orthe wavelength of the device. The degree of polarization was determinedby measuring the light emitted in the wafer plane. The in-plane emittedlight was collected with a lens and analyzed after an iris diaphragm bya Glan-Taylor prism and optical fiber spectrometer. The light wasfocused with a second lens on the optical fiber spectrometer. Theexperimental setup for measuring the polarization of the light isdiscussed more fully in T. Kolbe, A. Knauer, C. Chua, Z. Yang, SEinfeldt, P Vogt, N. M. Johnson, M. Weyers and M. Kneissl, Appl. Phys.Lett. 97, 171105 (2010), which is incorporated herein by reference.

The degree of polarization of various III-nitride light emitting regionalloys spanning InGaN, InAlGaN, and AlGaN emitting in the wavelengthrange λ=250 nm to λ=400 nm are shown in FIG. 4. The integratedin-plane-emitted TE and TM polarized electroluminescence lightintensities are denoted by I_(TE) and I_(TM), respectively. As expected,the TM polarized component becomes increasingly dominant at shorterwavelengths. The dashed line is a guide for the eye, and the solid linedenotes the zero degree of polarization point, where equal intensitiesof TE and TM polarized light are emitted. The measured data plottedalong the dashed line suggest a cross-over wavelength at around λ=295nm, below which the polarization becomes predominantly TM.

Some devices described herein that achieve enhanced degree ofpolarization, P_(D), include light emitting regions that arecompressively strained and/or include selected Al content in the barrierregions. The compressive strain of the crystalline lattice in theemitting region and/or Al content of the barriers serves to counteractthe shift in polarization toward TM polarized light that accompaniesincreased Al content in UV emitting devices. Light emitting diodesfabricated according to the techniques discussed in this disclosure arerepresented by data labeled B4745 in FIG. 4.

Embodiments of the invention are directed to devices having lightemitting regions that generate light having an enhanced degree ofpolarization, P_(D). As will be appreciated from FIG. 4, the enhancedP_(D) may be greater than 0 or in a range between about 0 to about 0.8when 200 nm≤λ<300 nm. For devices emitting light having wavelengths inthe range 300 nm≤λ<350 nm, an enhanced P_(D) may have a value greaterthan about 0.5 or in the range of about 0.5 to about 0.8. For devicesemitting light in the wavelength range 350 nm≤λ<400 nm, enhanced P_(D)may be greater than about 0.7 or in a range from about 0.7 to about 0.8.

The enhanced degree of polarization, P_(D), may include values in theregion above and to the left of line 410 in FIG. 4 or even values in theregion above and to the left of line 405. For example, the enhanceddegree of polarization may be expressed as:P _(D)>0.006λ−b for 200 nm≤λ≤400 nm.  [1]where b may be less or equal to than about 1.5 (corresponding to line405) or less than or equal to about 1 (corresponding to line 410).

FIG. 5 shows the polarization-resolved electroluminescence intensitiesfor B4745, an LED having a compressively strained active region emittingat about λ=250 nm. Unlike conventional devices that follow thepolarization ratio trend described earlier and illustrated by the pointsalong the dashed line of FIG. 4, the B4745 device shows strong relativeemission in the preferred TE-polarization direction. The enhancedTE-polarization was achieved by growing the light emitting region ofB4745 under compressive strain. Strong compressive strain in the lightemitting region alters the band structure, and can push the lightemission characteristics toward TE-polarization mode as can be observedfrom FIGS. 4 and 5.

FIG. 6 is a diagram of layers of a light emitting region 600. The lightemitting region 600 may comprise InAlN, AlGaN, and/or InAlGaN, forexample. The central quantum well (QW) layer 610 can be compressivelystrained which supports enhanced TE polarization. In this example, thelight emitting region 600 includes a central Al_(x)Ga_(1-x)N QW layer610 disposed between first and second barrier layers 620, 630. In FIG.6, the first barrier layer 620 has composition Al_(y)Ga_(1-y)N and thesecond barrier layer has composition Al_(z)Ga_(1-z)N, where y may beequal to z for some embodiments. The Al content in the barrier layers620, 630 serves to achieve confinement of the holes to the QW layer 610.The Al content in the barrier layers and/or the difference between theAl content of the barrier layers and the Al content of the QWs may alsoinfluence P_(D), as discussed below.

The strain, ε_(a) of the QW 610 of the light emitting region 600 may beexpressed:ε_(a)=(a _(strained) −a _(relaxed))/a _(relaxed)  [2]where a_(relaxed) is the in-plane lattice constant of fully relaxedAl_(x)Ga_(1-x)N and a_(strained) is the actual strained in-plane latticeconstant of the QW 610. Note that according to Equation 2, compressivestrain is a negative quantity. Calculations support the conclusion thatstrain in the active region less than (more compressive than) certainvalues can provide TE dominant light emission (P_(D)>0) and/or canachieve enhanced P_(D) values>0 as set forth in connection with FIG. 4.The strain that achieves enhanced P_(D) depends on the wavelength of theemitted light and also on the Al composition in the barrier layers 620,630.

The lines of FIG. 7 correspond to the strain that may be used to achievedominant TE polarization at the specified wavelength for 5 barrier layerAl compositions, y=0.65, 0.7, 0.8, 0.9, and 1.0. If the strain in theQW, ε_(a), is below the line then TE polarized emission is possible forthe Al composition of the barriers, y, indicated. The compressive strainthat supports TE polarized emission depends on the Al composition in thebarriers, y, where both barrier layers have the same Al composition.Table 1 summarizes, for various amounts of Al content in the barriers,y, the strain in the QW, ε_(a), that may be used to achieve dominant TEpolarized emission as a function of λ.

TABLE 1 y ε_(a) 0.65 <−0.0094 + 0.00023 * (λ(nm) − 249 nm) 0.7<−0.0088 + 0.00022 * (λ(nm) − 245 nm) 0.8 <−0.0079 + 0.00022 * (λ(nm) −240 nm) 0.9 <−0.00715 + 0.00023 * (λ(nm) − 235 nm)  1.0 <−0.00615 +0.00023 * (λ(nm) − 230 nm) 

Note that as defined herein, compressive strain is indicated by anegative strain, so strains that are less than values indicated in Table1 (more negative) correspond to a greater amount of compressive strain.

The light emitting device may be formed using a variety of substratematerials, such as sapphire, GaN, AlN, AlGaN, InAlGaN, Si, SiC and/orother materials. FIG. 7 and Table 1 may generally be applied to lightemitting regions for any substrate wherein structural relaxation occursand the active region does not necessarily have the lattice constant ofthe substrate. The relaxation can occur by the introduction ofdislocations during the growth of the light emitting layers (AlGaNlayers).

In some embodiments, AlN is used as the substrate for the light emittingdevice and the light emitting region includes at least oneAl_(x)Ga_(1-x)N region disposed between two Al_(y)Ga_(1-y)N regionswhere 1≥y>x. In these embodiments, the compressive strain of the lightemitting region that achieves enhanced P_(D) can be expressed as afunction of the difference between the lattice constant of the AlNsubstrate, a_(bulk-AlN), and the lattice constant of the Al_(x)Ga_(1-x)NQW, a_(strained) For example, in some embodiments, enhanced P_(D) may beachieved when a_(strained)-a_(bulk-AlN)<0.01 Å.

The lines of FIG. 8 correspond to the difference between the latticeconstant of the QWs and the lattice constant of the substrate,a_(strained)-a_(bulk-AlN), that achieves enhanced P_(D) at the specifiedwavelength for two barrier layer Al compositions, y=0.7 and y=1. Thestrain in the QWs of the light emitting region for enhanced P_(D) candepend on the aluminum content of the barrier layers, y, or thedifference between the Al molar fraction in the barriers, y, and the Alcontent, x, of the QWs. From FIG. 8, when y=1.0, enhanced P_(D), e.g.,TE dominant polarization, is achieved when the difference in latticeconstants is a_(strained)-a_(bulk-AlN)≤(λ(nm)−230)*0.0012 Å. When y=0.7,enhanced P_(D), e.g., TE dominant polarization, is achieved when thedifference in lattice constants isa_(strained)-a_(bulk-AlN)≤(λ(nm)−245)*0.0012 Å.

The Al content of the barrier, y, that can be used to achieve enhancedP_(D) for a given value of λ for an Al_(x)Ga_(1-x)N QW and AlN bulkcrystalline substrate can be expressed,y>1−0.02*{λ[nm]−230 [nm]−(a _(strained) [Å]−a _(bulk-AlN)[Å])/0.0012}.  [3]For the case that the growth is pseudomorphic, then(a_(strained)=a_(bulk-AlN)). In the pseudomorphic case, an Al content inthe barrier of at least y=0.7 may be used in a device designed to emitenhanced P_(D) light at a wavelength of around 245 nm. If somerelaxation takes place, so that a_(strained)-a_(bulk-AlN)>0, thenaccording to Equation 3 a somewhat larger barrier composition may beused. In some devices emission of enhanced P_(D) light may be achievedif y−x>0.05, but for devices designed to emit enhanced P_(D) light at awavelength λ below 245 nm it may be beneficial to increase the Alcontent in the barrier so that it meets both constraints A) y>x and B)y>0.7+0.02*(245−λ(nm)). Note that if x is 0.65 then λ would be about 245nm so constraint B stipulates that y>0.7 If x=0.625 then bothconstraints stipulate y>0.625. If x=0.6, then λ would be about 252 nmand constraint A stipulates that y>0.6. Thus, emission of shortwavelength TE polarized light can be achieved by using higher Alcomposition in the barriers. Somewhere between about λ=252 and λ=245 nmincreasing Al composition in the barriers contributes more substantiallyto TE polarization.

FIG. 9 illustrates a light emitting device 900 having a light emittingregion 905 comprising one or more compressively strained Al_(x)Ga_(1-x)Nlayers 910 disposed between Al_(y)Ga_(1-y)N barrier layers 920, 930.Heterostructures 940, 950 disposed on either side of the light emittingregion 910 enhance the crystal quality of the epitaxially grown layers910, 920, 930 of the light emitting region 905. A first heterostructure940 is formed over a substrate 960, and includes transition region 965.Note that in FIG. 9 a light emitting region 905 comprising a singlebarrier layer—QW—barrier layer structure is illustrated, the lightemitting region may include multiple QWs, each QW disposed betweenbarrier layers in a barrier layer—QW—barrier layer structure. Forexample, a light emitting diode may include 2, 3, 4, or more QWs.

Referring to the diagram of FIG. 10, the first heterostructure 1040 mayinclude one or more transition layers 1045 disposed between the lightemitting region 1005 and the substrate 1060. As previously discussed, asecond heterostructure (e.g., as indicated by 950 of FIG. 9) is grownover the light emitting region 1005. The transition layers of the firstheterostructure 1040 serve to further enhance crystal quality of thelight emitting device 1000. For example, the transition layers may beAlGaN/AlGaN short period superlattices or one or more graded AlGaNlayers that change in aluminum content going from the substrate towardthe active region. In some implementations, a graded AlGaN transitionlayer 1045 may be grown with a first surface 1046 towards the substrateand a second surface 1047 towards the light emitting region 1005,wherein the Al content changes (increases or decreases) from thetransition layer first surface 1046 to the transition layer secondsurface 1047. Optionally, a base layer 1065 may be present between thesubstrate 1060 and the transition layer 1045. The base layer 1065 canprovide a template upon which the first heterostructure is grown.Suitable materials for the base layer include Al_(zbase)Ga_(1-zbase)N,where zbase is between 0 and 1.

If the base layer is lower in Al content than the light emitting region,then the Al content in the transition layer may increase from the baselayer toward the light emitting region. If the base layer is higher Alcontent than the light emitting region, then the Al content in thetransition layer may decrease from the base layer toward the lightemitting region.

In some configurations, the substrate 1060, e.g., a bulk AlN substrate,may be too optically absorbing to allow efficient extraction of lightfrom the bottom of the device 1000. In these configurations, lightextraction can be enhanced by growing an optional layer 1061 on the AlNsubstrate 1060. For example, the optional layer 1061 may be a relativelythick pseudomorphic AlN layer grown homoepitaxially on the bulk AlNsubstrate 1060. The homoepitaxially grown AlN layer 1061 isnon-absorbing at the desired wavelength of the device 1000. In somecases, the optional layer 1061 may be an epitaxially grown layercomprising AlGaN, such as an AlGaN layer or an AlGaInN layer. Most orall of the substrate 1060 or all of the substrate 1060 and a portion ofthe optional layer 1061 may be etched away, e.g., as indicated by dashedlines 1062 and 1063, respectively. The thickness, t, of the portion ofthe homoepitaxially grown AlN layer 1061 that remains with the device1000 after the etching is sufficient to maintain the compressive strainthat provides dominant TE polarized light emission from the lightemitting region 1005. For example, the compressive strain for a devicethat produces dominant TE polarized light and having a removed AlNsubstrate may satisfy the equation:a_(strained)-a_(bulk-AlN)≤(λ(nm)−230)*0.0012 Å. When an epitaxial AlGaNor AlGaInN optional layer is used, the optional layer 1061 may berelatively thin and the amount of Ga and/or In may be relatively smallin order to reduce wafer bowing once the bulk AlN substrate is removed.

FIG. 11 shows the various layers and layer compositions of an example ofa light emitting device 1100. In this example, the device includes abulk crystalline AlN substrate 1160. Epitaxially grown on the bulk AlNsubstrate is a transition region 1165 comprising two 38 nm-thick shortperiod superlattice (SPSL) sections 1166, 1167. The first,substrate-side, section contains an average aluminum molar fraction of90%. The second section is grown above the first section and contains anaverage aluminum content of 80%. The superlattice in the second sectionhas a thinner AlN component than the first section to produce the loweraverage aluminum alloy composition. The transition layer 1165 serves totransition the Al content of the device from the (relatively high Alcontent) AlN substrate 1160 toward the (lower Al content) light emittingregion 1110 to maintain pseudomorphic growth of AlGaN light emittingregion. The light emitting region 1110 is thus compressively strainedtowards conformation with the lattice of the AlN substrate 1160. In thisexample, the transition layer 1165 decreases in Al content from the AlNsubstrate 1160 towards the light emitting region 1110.

A 1900 nm Al_(0.70)Ga_(0.30)N silicon doped n-contact layer 1170 isdisposed on the transition layer 1165 followed by an n-cladding layer1171 comprising a 650 nm Al_(0.70)Ga_(0.30)N Si doped superlattice. Thelight emitting region 1110 and an electron blocking layer (EBL) 1150 aredisposed between an n-side separate confinement heterostructure (SCH)1172 and p-side current spreading layer 1155. In the example of FIG. 11,the n-side SCH 1172 comprises a 30 nm Al_(0.68)Ga_(0.32)N layer, and thep-side current spreading layer 1155 comprises a 65 nm layer of Mg dopedAl_(0.33)Ga_(0.67)N. In this example, the EBL 1150 is a 20 nm Mg dopedAl_(0.83)Ga_(0.17)N layer. The light emitting layer 1110 comprises threeAl_(x)Ga_(1-x)N quantum well layers alternating with AlyGa1-yN barriers.The Al content, x, of the QWs is sufficient to achieve a centralwavelength of λ=253 nm. The Al content, y, of the barriers, aspreviously discussed, is selected to compressively strain the QWs sothat emission of TE-polarized light from the light emitting region 1110is predominant. The light emitting device 1100 includes ap-cladding/current spreading layer 1156 comprising a 150 nm AlGaN/AlGaNMg doped superlattice having an average composition ofAl_(0.33)Ga_(0.67)N disposed above the current spreading layer 1155. A20 nm p-contact layer 1180 comprising Mg doped GaN is disposed on thecurrent spreading layer 1156.

As previously discussed, the transition layers are included in thedevice structure to enhance crystal quality of the device. FIGS. 12 and13 show surface atomic force microscopy (AFM) scans of theheterostructure in FIG. 11 grown up to the n-contact layer, without(FIG. 12) and with (FIG. 13) transition layers. The structure shown inFIG. 12 without the transition layer is optically hazy, is rough, andhas many more hexagonal pits compared to the structure with transitionlayers shown in FIG. 13. Using transition layers allows for the growthof device structures (up to the n-contact layer) having a surfaceroughness of less than about 15 nm, or less than about 10 nm or evenless than about 5 nm.

FIG. 15 shows an X-ray reciprocal space map of the structure 1400 shownin FIG. 14. FIG. 14 illustrates a Al_(0.7)Ga_(0.3)N layer 1470 grownover a bulk crystalline AlN substrate 1460. Two 38 nm-thick transitionlayers comprising short period superlattices (SPSLs) 1465, 1466 aredisposed between the AlN substrate 1460 and the Al_(0.7)Ga_(0.3)N layer1470. The first, substrate-side, SPSL 1465 contains an average aluminummolar fraction of 90%. The second SPSL 1466 is grown above the firstSPSL 1465 and contains an average aluminum content of 80%.

FIG. 15 is the reciprocal space map acquired by x-ray diffractionanalysis of structure 1400. The peaks for the transition layers and theAlGaN layer all line up along the same Qx value as that of the substrateindicating a close lattice match in the layers. This characteristicdemonstrates pseudomorphic growth resulting in an amount of compressivestrain that produces TE-dominant LED emission. For example, in somecases, TE polarized light is dominant when the variation in reciprocallattice values of the AlN substrate and the light emitting region isless than about 1.5%.

In addition to λ=250 nm light emitting diodes, shown schematically inFIG. 11 and having the photoluminescence plot provided in FIG. 5,additional devices having active layers operating at λ=279 and at λ=290nm were designed, grown, fabricated, and evaluated. Like the λ=250 nmdemonstration, these longer wavelength devices were grown under highcompressive strain on AlN substrates. However, they use a differenttransition layer comprising a graded AlGaN layer with aluminumcomposition that monolithically decreases from 100% to 70% instead ofthe superlattice transition layers illustrated in FIG. 11.Photoluminescence data from these longer wavelength devices show thedesired TE-dominant light emission. Photo-pump lasers made from thesedevices emit light with TE-dominant polarization both below and abovethe lasing threshold. FIG. 16 shows the polarization-resolved spectrumof the λ=279 nm laser and FIG. 17 shows the polarization-resolvedspectrum of the λ=290 nm laser.

Compressive strain in the light emitting region of a light emittingdevice can be designed to enhance TE-polarized emission. Additionally,the light emitting region quantum well thickness, barrier thickness, andbarrier composition also influence the band structure and the nature ofenergy transitions. The well width, barrier width, and barriercomposition can also be adjusted to tune the polarization ratio. UVlight emitting structures that exhibit enhanced P_(D) light emissionhave been attained by growing the light emitting region undercompressive strain and/or tuning the barrier Al content. Despite thelarge strain, crystal quality may be maintained by using superlatticesand/or graded transition layers between the heterostructure and thesubstrate.

FIG. 18 is a flow diagram of a method of forming a light emitting devicecapable of emitting light having enhanced degree of polarization, P_(D).A first heterostructure is grown 1810 on a substrate. A light emittingregion 1845 is grown over the first heterostructure. The light emittingregion is grown so that the compressive strain in the light emittingregion satisfies the inequality ε_(a)<−0.00615+0.00023*(λ(nm)−230 nm)for y≤1. A second heterostructure is grown over the light emittingregion.

Example

FIG. 18 shows the layers 1805-1865 of an example light emitting diode1800 emitting light at a central wavelength, λ=291 nm, and with enhancedP_(D) light emission as discussed herein. The device 1800 was producedby growing layers 1810-1865 by metal organic chemical vapor deposition(MOCVD) on a bulk crystalline AlN substrate 1805. A 54 nm initiation orbase layer 1810 comprising undoped MOCVD grown AlN was grown on the bulkAlN substrate 1805. Defect reduction layers/transition layers (DRL1 andDRL2) comprise a 60 pair AlN/GaN superlattice having an averagecomposition of Al_(0.89)Ga_(0.11)N 1815 and thickness 144 nm 1815 and a101 pair AlN/GaN superlattice having an average composition ofAl_(0.78)Ga_(0.22)N and a thickness of 271 nm 1820. A 315 nm n-contactbase layer 1825 comprising Al_(0.74)Ga_(0.26)N and a Si doped 484 nmAl_(0.74)Ga_(0.26)N n-contact layer 1830 were grown over the transitionlayers as depicted in FIG. 18. Next grown was an n-cladding layer 1835comprising a Si doped 78-pair AlGaN superlattice having a totalthickness of 504 nm and an average composition of Al_(0.74)Ga_(0.26)N.An n-waveguide 1840 comprising 33 nm Si doped Al_(0.59)Ga_(0.41)N wasgrown over the n-cladding superlattice. The light emitting region 1845comprises three 3.8 nm QWs comprising Al_(0.44)Ga_(0.56)N layer disposedbetween 5.3 nm barrier layers comprising Al_(0.57) Ga_(0.43)N. Thep-side of the device includes a 5.9 nm p-waveguide 1850 comprisingAl_(0.59)Ga_(0.41)N and a 367 nm p-cladding layer 1855. The p-claddinglayer 1855 comprises a 200 pair AlGaN/AlGaN superlattice having anaverage composition Al_(0.85)Ga_(0.15)N. The Al content is graded inlayer 1860 from molar fraction 0.65 to 0. Layer 1865 is the p-contactlayer. Table 2 below provides a summary of parameters used for growinglayers 1810-1865. In Table 2, the following abbreviations are used:DRL=defect reduction layer, BA=barrier, QW=quantum well,TMG=trimethylgallium, TEG=triethylgallium, TMA=trimethylaluminum,CP2Mg=Bis(cyclopentadienyl)magnesium, sccm=standard cubic centimetersper minute.

TABLE 2 Time Temp Pressure TMG/TEG TMA NH3 SiH4 CP2Mg Layer (sec) (° C.)(torr) (sccm) (sccm) (sccm) (sccm) (sccm) Carrier 1810 AlN undoped 9001125 200 10 4 H 1815 DRL 1 AlN 31 1100 200 10 4 H 1815 60x DRL 1 GaN 51100 200 5 4 H 1820 DRL 2 AlN 12 1100 200 10 4 H 1820 101x  DRL 2 GaN 51100 200 5 4 H 1825 nCont, undoped 1800 1100 200 4 95 4 H 1830 nCont2700 1100 200 4 95 4 2 H 1835 nClad, xHi 16 1100 200 4 100 4 2 H 183568x nClad, xLo 20 1100 200 4 90 4 2 H 1835 nClad, xHi 16 1100 200 4 1004 2 H 1835 10x nClad, xLo 20 1100 200 4 90 4 2 H 1840 nGuide 1380 900700 64 10 5 0.8 N 1845 BA:un 55 900 700 64 8 5 N 1845 BA:Si 111 900 70064 8 5 1 N 1845 BA:un 55 900 700 64 8 5 N 1845 QW1 160 900 700 64 3.5 5N 1845 BA:un 55 900 700 64 8 5 N 1845 BA:Si 111 900 700 64 8 5 1 N 1845BA:un 55 900 700 64 8 5 N 1845 QW2 160 900 700 64 3.5 5 N 1845 BA:un 55900 700 64 8 5 N 1845 BA:Si 111 900 700 64 8 5 1 N 1845 BA:un 55 900 70064 8 5 N 1845 QW3 160 900 700 64 3.5 5 N 1845 BA:un 55 900 700 64 8 5 N1845 BA:Si 111 900 700 64 8 5 N 1845 BA:un 55 900 700 64 8 5 N 1850pGuide 246 900 700 64 10 5 N 1855 pClad, xHi 50 900 200 3 4 H 1855 200x pClad, xLo 25 900 200 0.5 3 4 25 H 1860 pGrad AlGaN 5 900 200 5 60 4 100H 1860 ramp pGrad AlGaN 333 900 200 5 60 → 0 4 100 H 1865 GaN:Mg 100 900200 5 4 100 H

Light emitting devices described in examples provided herein are basedon compressively strained III-nitride material systems. FIG. 19 shows arelationship between the Al content of Al_(x)Ga_(1-x)N and emissionwavelength. It will be appreciated that, using the relationship providedin FIG. 19, for example, the wavelength ranges provided herein may beviewed in terms of the Al content in the light emitting region whichcorresponds to these wavelength ranges. For example, the wavelengthrange between 200 nm and 400 nm generally corresponds to an Al molarfraction range between about 0 and 1; the wavelength range between 200nm and 300 nm generally corresponds to an Al molar fraction rangebetween about 0.32 and 1. Note that growing pseudomorphicAl_(x)Ga_(1-x)N on AlN can become more difficult as x is reduced.However, low x AlGaN could relax (so that it is no longer pseudomorphicto AlN) and still be compressively strained, producing light having anenhanced degree of polarization as described herein. Light emittingdevices according to some embodiments may be described in terms of thecompressive strain and Al content of the active region. For example, alight emitting device according to the approaches herein may comprise atleast one compressively stained layer of Al_(x)Ga_(1-x)N, wherein0.5<x<0.8, and wherein a compressive strain, ε_(a), in the compressivelystrained layer satisfies the inequality ε_(a)<−0.00615.

A number of values and ranges are provided in various aspects of theimplementations described. These values and ranges are to be treated asexamples only, and are not intended to limit the scope of the claims.For example, embodiments described in this disclosure can be practicedthroughout the disclosed numerical ranges. In addition, a number ofmaterials are identified as suitable for various facets of theimplementations. These materials are to be treated as exemplary, and arenot intended to limit the scope of the claims. The foregoing descriptionof various embodiments has been presented for the purposes ofillustration and description and not limitation. The embodimentsdisclosed are not intended to be exhaustive or to limit the possibleimplementations to the embodiments disclosed. Many modifications andvariations are possible in light of the above teaching.

The invention claimed is:
 1. A nitride-semiconductor light emittingdevice comprising: a substrate; a first heterostructure; a secondheterostructure; a light emitting region comprising: barrier layerscomprising AlGaN; and one or more quantum wells comprising AlGaN, eachquantum well respectively disposed between two of the barrier layers,the light emitting region disposed between the first and secondheterostructures and configured to emit light having a centralwavelength, λ, and a degree of polarization, P_(D), where P_(D)>0.006λ−bfor 200 nm≤λ≤400 nm, wherein b≤1.5, wherein the first heterostructure isdisposed between the light emitting region and the substrate andincludes a transition layer comprising a two section AlN/GaNsuperlattice, the two section AlN/GaN superlattice having a firstsurface near the substrate and a second surface near the light emittingregion, the two section superlattice configured to transition Al contentof the device from a higher Al content near the first surface to a lowerAl content near the second surface.
 2. The device of claim 1, wherein bis about
 1. 3. The device of claim 1, wherein the light emitting regioncomprises at least one of GaN, InAlN, AlGaN, InGaN and InAlGaN.
 4. Thedevice of claim 1, further comprising a substrate, the substratecomprising at least one of sapphire, GaN, AlN, AlGaN, InAlGaN, Si, andSiC.
 5. The device of claim 1, further comprising a bulk crystalline AlNsubstrate.
 6. The device of claim 5, wherein: the AlN substrate has alattice constant, a_(bulk-AlN); the light emitting region comprises atleast one Al_(x)Ga_(1-x)N region; and the at least one Al_(x)Ga_(1-x)Nregion has an in-plane lattice constant a_(strained) anda_(strained)−a_(bulk-AlN)(λ(nm)−230)*0.0012 Å.
 7. The device of claim 6,wherein the Al_(x)Ga_(1-x)N region is disposed between twoAl_(y)Ga_(1-y)N regions, where 0≤y≤1, y>x and y>0.7+0.02*(245−λ(nm)). 8.The device of claim 1, wherein the light emitting region comprises atleast one Al_(x)Ga_(1-x)N layer and wherein the strain ε_(a) in theAl_(x)Ga_(1-x)N layer satisfies the inequalityε_(a)<−0.0079+0.00022*(λ(nm)−240 nm).
 9. The device of claim 1, whereinλ≤300 nm and P_(D)≥0.
 10. The device of claim 1, further comprising abase layer disposed between the substrate and the first heterostructure,wherein the base layer comprises Al_(zbase)Ga_(1-zbase)N, where zbase isbetween 0 and
 1. 11. The device of claim 1, wherein: a first section ofthe AlN/GaN superlattice has an average aluminum molar fraction of about90%; and a second section of the AlN/GaN superlattice has an averagealuminum, molar fraction of about 80%.
 12. The device of claim 1,wherein a transition layer surface closest to the light emitting regionhas a surface roughness less than about 10 nm.
 13. A nitridesemiconductor light emitting device, comprising: a bulk crystalline AlNsubstrate; a light emitting region comprising AlGaN grown above the bulkcrystalline AlN substrate; and a first heterostructure disposed betweenthe light emitting region and the bulk crystalline AlN substrate, thefirst heterostructure including a transition layer comprising an AlN/GaNsuperlattice, wherein variation in reciprocal lattice values of the bulkcrystalline AlN substrate and the light emitting region is less thanabout 1.5% and wherein the light emitting region is configured to emitlight having a central wavelength, λ, and a degree of polarization,P_(D), where P_(D) is greater than 0.006λ−1.5 for 200≤λ≤400 nm, theAlN/GaN superlattice having a first surface near the substrate and asecond surface near the light emitting region, an Al content of theAlN/GaN superlattice changing from a higher Al content near the firstsurface to a lower Al content near the second surface.
 14. The device ofclaim 13, wherein: the AlN substrate has a lattice constant,a_(bulk-AlN); the light emitting region comprises at least oneAl_(x)Ga_(1-x)N region; and the at least one Al_(x)Ga_(1-x)N region hasan in-plane lattice constant a_(strained) that satisfies the inequalitya_(strained)−a_(bulk-AlN)≤(λ(nm)−230)*0.0012 Å.
 15. The device of claim14, wherein the Al_(x)Ga_(1-x)N region is disposed between twoAl_(y)Ga_(1-y)N regions, and wherein 0≤y≤1, y>x andy>0.7+0.02*(245−λ(nm)).